The present invention relates to an image printing apparatus, control method therefor, storage medium, and program and, more particularly, to a uniform image printing method in an ink-jet printing apparatus for printing information by discharging ink to a printing member.
A printing apparatus used to print an image or the like in a printer, copying machine, facsimile apparatus, or the like, or a printing apparatus used as a print output device in a workstation or a composite electronic device including a computer, word processor, and the like prints an image or the like on a printing member (to be also referred to as a printing medium hereinafter) such as a sheet or plastic thin plate on the basis of image information (including all pieces of output information such as character information).
Printing apparatuses can be classified into an ink-jet type, wire dot type, thermal type, laser beam type, and the like depending on their printing methods.
Of these printing apparatuses, the ink-jet printing apparatus (to be referred to as an ink-jet printer hereinafter) prints information by discharging ink onto a printing medium from a printhead or the like. Compared to other printing types, the ink-jet printer has various advantages such as easy implementation of high resolution, high speed, low noise, and low cost.
In recent years, color outputs such as a color image become more and more important, and a variety of color ink-jet printers with high quality equivalent to a silver halide photograph have been developed.
To increase the printing speed, the ink-jet printer adopts a printhead on which pluralities of ink orifices and liquid channels are integrated as a printhead (to be also referred to as a multihead hereinafter) on which a plurality of printing elements are integrally aligned. To output color images, the ink-jet printer generally comprises a plurality of multiheads.
FIG. 1 is a view showing the main part of a general ink-jet printer for printing information on a sheet surface by using the multihead.
In FIG. 1, reference numerals 1101 denote ink-jet cartridges. These ink-jet cartridges are made up of ink tanks which store four color inks, i.e., black, cyan, magenta, and yellow inks, and multiheads 1102 corresponding to the respective inks.
FIG. 2 is a schematic view showing orifices (to be also referred to as nozzles hereinafter) for one color arranged in the multihead 1102 when viewed from a Z direction in FIG. 1.
In FIG. 2, reference numerals 1201 denote D nozzles aligned at a density of D nozzles per inch (D dpi) in the multihead 1102. Even-numbered nozzles out of d aligned nozzles will be called Even nozzles, and odd-numbered nozzles will be called Odd nozzles.
In FIG. 1, reference numeral 1103 denotes a sheet supply roller, which rotates together with an auxiliary roller 1104 in a direction indicated by an arrow in FIG. 1 while clamping a printing medium P between them, and conveys the printing medium P in the Y direction (subscanning direction, convey direction, and sheet supply direction).
Reference numerals 1105 denote a pair of sheet feed rollers, which feed a printing medium. Similar to the rollers 1103 and 1104, the pair of rollers 1105 rotate while clamping the printing medium P. The rotational speed of the rollers 1105 is set lower than that of the sheet supply roller 1103 to apply tension to the printing medium.
Reference numeral 1106 denotes a carriage which supports the four ink-jet cartridges 1101 and scans them at the same time as printing. The carriage 1106 stands by at a home position h represented by a broken line in FIG. 1 during an idle period of printing or in recovery processing of the multihead 1102.
If the carriage 1106 at the home position h receives a printing start instruction before the start of printing, the carriage 1106 moves in the X direction (main scanning direction). D/D-inch wide printing is done on a; sheet surface by the D nozzles 1201 of the multihead 1102 which are aligned at a density of D nozzles per inch. During an interval between the end of the first printing and the start of the second printing, the sheet supply roller 1103 rotates in the direction indicated by the arrow to supply the sheet in the Y direction by a D/D-inch width.
D/D-inch wide printing by the multiheads 1102 (information is printed on a 1-inch wide portion of a printing medium by using D nozzles) and sheet supply are repeated every main scanning of the carriage 1106, completing, e.g., printing of one page. This printing mode will be called a 1-pass printing mode.
Another printing mode will be described. If the carriage 1106 at the home position h receives a printing start instruction before the start of printing, the carriage 1106 moves in the X direction (e.g., forward direction of main scanning). D/D-inch wide printing is done on a sheet surface by the D nozzles 1201 of the multihead 1102 which are aligned at a density of D nozzles per inch.
Dots printed by this scanning form an image of specified image data which is interlaced into almost half by a predetermined pattern. During an interval between the end of the first printing and the start of the second printing, the sheet supply roller 1103 rotates in the direction indicated by the arrow to supply the sheet in the Y direction by a D/2D-inch width.
In the second scanning, the carriage 1106 is scanned in a direction (e.g., backward direction of main scanning) opposite to that in the first printing. Images are printed in accordance with respective patterns, completing printing in regions corresponding to respective nozzles. This printing mode will be called a 2-pass printing mode. M (xe2x89xa72)-pass printing will be generally called a multipass printing mode.
As a color printer, the ink-jet printer can optimally print a photographic image at high quality in the multipass printing mode.
However, a uniform image may not be obtained owing to the discharge direction of ink droplets discharged from nozzles, or ink droplets (to be referred to as satellites) which are separated from main droplets in discharge and are smaller than main droplets.
Especially when the discharge direction changes in the main scanning direction between Even and Odd nozzles of d aligned nozzles, the landing positions of satellites on the sheet surface change, failing to forming a uniform image.
A case in which a uniform image cannot be obtained due to satellites and different discharge directions of Even and Odd nozzles will be explained in detail with reference to the accompanying drawings.
FIGS. 3A to 3C are views showing the landing positions of a main droplet and satellite on a sheet surface serving as a printing medium in an ink droplet discharge direction.
FIG. 3A is a schematic view showing the landing positions of a main droplet and satellite when the ink droplet discharge direction is perpendicular to the sheet surface.
FIG. 3B is a schematic view showing the landing positions of a main droplet and satellite when the ink droplet discharge direction inclines to the carriage traveling direction.
FIG. 3C is a schematic view showing the landing positions of a main droplet and satellite when the ink droplet discharge direction inclines to a direction opposite to the carriage traveling direction.
In FIGS. 3A to 3C, reference numeral 1301 denotes a main droplet; 1302, a satellite; 1303, a carriage traveling direction; and 1304, a discharge inclination direction.
The landing positions of the main droplet and satellite when the ink droplet discharge direction is perpendicular to the sheet surface serving as a printing medium, i.e., the ink droplet discharge direction does not incline to the carriage traveling direction will be explained with reference to FIG. 3A.
In FIG. 3A, a comparison between the discharge speeds of the main droplet 1301 and satellite 1302 discharged from a nozzle reveals that the discharge speed of the main droplet 1301 is generally higher than that of the satellite 1302. A time taken to discharge ink and land it on the printing medium is longer for the satellite 1302 than for the main droplet 1301. The satellite 1302 lands on the sheet surface serving as a printing medium after the main droplet 1301 lands on it. A predetermined time is required for landing the satellite 1302 after the main droplet 1301 lands.
The main droplet 1301 and satellite 1302 are discharged while the carriage 1106 moves. The carriage speed in the carriage traveling direction is added to the discharge speeds of the main droplet 1301 and satellite 1302.
For this reason, the landing points of the main droplet 1301 and satellite 1302 on the sheet surface serving as a printing medium differ from each other. The satellite 1302 lands in the traveling direction of the carriage 1106 with respect to the landing position of the main droplet 1301 shown in FIG. 3A.
The landing positions of the main droplet and satellite when the ink droplet discharge direction inclines to the carriage traveling direction 1303 with respect to the sheet surface serving as a printing medium will be described with reference to FIG. 3B.
In FIG. 3B, the ink droplet discharge direction inclines to the carriage traveling direction 1303. The speed of the satellite 1302 in the carriage traveling direction 1303 is higher than the speed when the ink droplet discharge direction is perpendicular to the sheet surface (FIG. 3A). The satellite 1302 lands at a position shown in FIG. 3B more apart from the main droplet 1301 than the landing point of the satellite 1302 shown in FIG. 3A.
The landing positions of the main droplet and satellite when the ink droplet discharge direction inclines to a direction opposite to the carriage traveling direction 1303 with respect to the sheet surface serving as a printing medium will be described with reference to FIG. 3C.
In FIG. 3C, the ink droplet discharge direction inclines to a direction opposite to the carriage traveling direction 1303. The speed of the satellite 1302 in the carriage traveling direction is lower than the speed when the ink droplet discharge direction is perpendicular to the sheet surface (FIG. 3A). The satellite 1302 lands at a position nearer the main droplet 1301 than the landing point of the satellite 1302 shown in FIG. 3A, or on a side opposite to the carriage traveling direction. FIG. 3C shows a case in which the satellite 1302 lands at almost the same position as that of the main droplet 1301.
The printing quality problem in the multipass printing mode executed in a conventional ink-jet printer will be described with reference to FIGS. 4A to 4D and 5A to 5D.
In FIGS. 4A to 4D and 5A to 5D, the ink droplet discharge direction of an Even nozzle inclines to the main scanning direction, and that of an Odd nozzle inclines to a direction opposite to the main scanning direction. The problem is the same regardless of whether the inclination directions are reversed.
Examples in FIGS. 4A to 4D will be explained.
FIGS. 4A to 4D are schematic views each showing a case in which a 1/D-inch region is defined as a unit printing pixel (area surrounded by dotted line) in the multipass printing mode for performing 4-pass printing, four dots are printed in the unit printing pixel, and a printing medium is supplied by an even multiple of 1/D inch. In this case, the following four patterns are conceivable.
FIG. 4A is a schematic view showing a dot pattern when the first pass printing starts by an Even nozzle while the carriage travels in the main scanning (X) direction.
FIG. 4B is a schematic view showing a dot pattern when the first pass printing starts by an Odd nozzle while the carriage travels in the main scanning (X) direction.
FIG. 4C is a schematic view showing a dot pattern when the first pass printing starts by an Even nozzle while the carriage travels in a direction opposite to the main scanning (X) direction.
FIG. 4D is a schematic view showing a dot pattern when the first pass printing starts by an Odd nozzle while the carriage travels in a direction opposite to the main scanning (X) direction.
In FIGS. 4A to 4D, reference numeral 401 denotes a first pass printing dot; 402, a second pass printing dot; 403, a third pass printing dot; and 404, a fourth pass printing dot. In practice, four, first to fourth pass printing dots overlap each other and are printed. In FIGS. 4A to 4D, one main droplet and one satellite are formed, which express the tonality of the unit printing pixel. The following description adopts the above expression for descriptive convenience.
The dot patterns in FIGS. 4A to 4D appear on a printing medium as follows. That is, the dot patterns in FIGS. 4A and 4B (or FIGS. 4C and 4D) alternately appear every 1/D inch in the sheet supply direction.
In FIGS. 4A to 4D, arrows (← andxe2x86x92) illustrated in the unit printing pixel represent carriage traveling directions in respective pass printing operations. E represents a dot printed by an Even nozzle, and O represents a dot printed by an Odd nozzle. The printing quality problem in the conventional multipass printing mode will be explained in detail with reference to FIGS. 4A to 4D.
The pattern in FIG. 4A will be first described.
In FIG. 4A, the first pass printing is done by an Even nozzle while the carriage moves in the main scanning (X) direction. A main droplet 301 and satellite 302 land at distant positions.
The second pass printing is performed after a sheet is supplied by an even multiple of 1/D inch. This printing is also done by an Even nozzle. Since printing is performed while a carriage 106 moves in a direction opposite to the X direction, the main droplet 301 and satellite 302 land at close positions. The third and fourth pass printing operations are executed similarly to the first and second pass printing operations, thereby printing dots with a dot pattern as shown in FIG. 4A.
As shown in FIG. 4A, all the dots are printed by Even nozzles within the unit printing pixel when the first pass printing starts by an Even nozzle while the carriage 106 travels in the main scanning direction
The pattern in FIG. 4B will be described.
In FIG. 4B, the first pass printing is done by an Odd nozzle while the carriage moves in a direction opposite to the main scanning direction (X). The main droplet 301 and satellite 302 land at distant positions.
The second pass printing is performed after a sheet is supplied by an even multiple of 1/D inch. This printing is also done by an Odd nozzle. Since printing is performed while the carriage 106 moves in the X direction, the main droplet 301 and satellite 302 land at close positions.
The third and fourth pass printing operations are executed similarly to the first and second pass printing operations, thus printing dots with a dot pattern as shown in FIG. 4B.
As shown in FIG. 4B, all the dots are printed by Odd nozzles within the unit printing pixel when the first pass printing starts by an Odd nozzle while the carriage 106 travels in the main scanning direction (X).
Similarly in FIG. 4C or 4D, all the dots within the unit printing pixel are printed by only Even or Odd nozzles.
If all the printing pixels are printed by Odd or Even nozzles, as shown in FIGS. 4A to 4D, the discharge characteristic may change such that the ink discharge amount differs between Odd and Even nozzles. The printing ink amount is large in a given pixel but small in another pixel. As a result, a visually nonuniform image is printed.
The patterns of FIG. 4A and FIG. 4B (or FIG. 4C and FIG. D) alternately appear every 1/D inch in the sheet supply direction. In other words, pixels (pixels as shown in FIG. 4A) in which satellites appear on the right of main droplets, and pixels (pixels as shown in FIG. 4B) in which satellites appear on the left of main droplets alternately appear every 1/D inch in the sheet supply direction. In other words, the satellite 302 alternately lands on the right and left of the main droplet 301 every 1/D inch. This leads to a visually nonuniform image.
Examples in FIGS. 5A to 5D will be explained.
FIGS. 5A to 5D are schematic views each showing a case in which a 1/D-inch region is defined as a unit printing pixel(area surrounded by dotted line) in the multipass printing mode for performing 4-pass printing, four dots are printed in the unit printing pixel, and a printing medium is supplied by an odd multiple of 1/D inch. In this case, the following four patterns are conceivable.
Similar to FIGS. 4A to 4D, FIGS. 5A to 5D show four dots as if they landed at different positions within a unit printing pixel for descriptive convenience. In practice, the four dots land at almost the same point within the unit printing pixel. The appearance of the dot patterns in FIGS. 5A to 5D is the same as that in FIGS. 4A to 4D. The dot patterns in FIGS. 5A and 5B (or FIGS. 5C and 5D) alternately appear every 1/D inch in the sheet supply direction.
FIG. 5A is a schematic view showing a dot pattern when the first pass printing starts by an Even nozzle while the carriage travels in the X direction.
FIG. 5B is a schematic view showing a dot pattern when the first pass printing starts by an Odd nozzle while the carriage travels in the X direction.
FIG. 5C is a schematic view showing a dot pattern when the first pass printing starts by an Even nozzle while the carriage travels in a direction opposite to the X direction.
FIG. 5D is a schematic view showing a dot pattern when the first pass printing starts by an Odd nozzle while the carriage travels in a direction opposite to the X direction.
In FIGS. 5A to 5D, reference numeral 401 denotes a first pass printing dot; 402, a second pass printing dot; 403, a third pass printing dot; and 404, a fourth pass printing dot. Arrows (← andxe2x86x92) illustrated in the unit printing pixel represent carriage traveling directions in respective pass printing operations. E represents a dot printed by an Even nozzle, and O represents a dot printed by an Odd nozzle. The reference numerals denote the same parts as in FIGS. 4A to 4D, and a repetitive description thereof will be omitted. The discharge inclinaptions of Odd and Even nozzles are also the same as those in FIGS. 4A to 4D.
The printing quality problem in the conventional multipass printing mode will be explained in detail with reference to FIGS. 5A to 5D.
The pattern in FIG. 5A will be first described.
In FIG. 5A, the first pass printing is done by an Even nozzle while the carriage moves in the main scanning (X) direction. The main droplet 301 and satellite 302 land at distant positions.
The second pass printing is performed after a sheet is supplied by an odd multiple of 1/D inch. This printing is done by an Odd nozzle. Since printing is performed while the carriage moves in a direction opposite while the X direction, the main droplet 301 and satellite 302 land at distant positions.
The third and fourth pass printing operations are executed similarly to the first and second pass printing operations, thereby printing dots with a dot pattern as shown in FIG. 5A.
As shown in FIG. 5A, all the dots are alternately printed using Odd and Even nozzles within the unit printing pixel when the first pass printing starts by an Even nozzle while the carriage 106 travels in the main scanning direction (X).
The pattern in FIG. 5B will be described.
In FIG. 5B, the first pass printing is done by an Odd nozzle while the carriage moves in the main scanning direction (X). The main droplet 301 and satellite 302 land at close positions.
The second pass printing is performed after a sheet is supplied by an odd multiple of 1/D inch. This printing is done by an Even nozzle. Since printing is performed while the carriage 106 moves in a direction opposite to the X direction, the main droplet 301 and satellite 302 land at close positions.
The third and fourth pass printing operations are executed similarly to the first and second pass printing operations, thus printing dots with a dot pattern as shown in FIG. 5B.
As shown in FIG. 5B, all the dots are alternately printed by Odd and Even nozzles within the unit printing pixel when the first pass printing starts by an Odd nozzle while the carriage 106 travels in the main scanning direction (X).
Although a description of the patterns in FIGS. 5C and SD will be omitted, all the dots within the unit printing pixel are alternately printed by Odd and Even nozzles, similar to FIGS. 5A and 5B.
That is, printing is achieved by supplying a sheet by an odd multiple of 1/D inch, as shown in FIGS. 5A to 5D. This prevents printing of all the unit printing pixels by only Odd or Even nozzles.
However, the patterns of FIG. 5A and FIG. 5B (or FIG. 5C and FIG.5D) alternately appear every 1/D inch in the sheet supply direction. The satellite 302 alternately lands on the right and left of the main droplet 301 every 1/D inch. In other words, pixels (pixels as shown in FIG. 5A) in which satellites appear on the right and left of main droplets, and pixels (pixels as shown in FIG. 5B) in which no satellite appears alternately appear every 1/D inch in the sheet supply direction. A visually nonuniform image is undesirably printed.
As described above, when a conventional ink-jet printer for repetitively scanning a printhead in the main scanning direction and a printing medium in the subscanning direction and forming an image by multipass (two or more passes) printing uses a multihead with a nozzle interval of 1/D inch and has different discharge characteristics between Odd and Even nozzles, this printer prints a visually nonuniform image by repetitively supplying a sheet by an even or odd multiple of 1/D inch.
The present Invention has been made to overcome the conventional drawbacks, and has as its object to provide an image printing apparatus capable of printing a uniform, high-quality image while avoiding printing of a visually nonuniform image in multipass printing of two or more passes, a control method therefor, storage medium and program.
To achieve the above object, an image forming apparatus according to an aspect of the present invention has the following arrangement. That is, an image printing apparatus which prints an image by multipass printing in which a printhead having a plurality of nozzles that are aligned at a predetermined nozzle pitch and discharge ink droplets is scanned on a printing medium in a direction cross to an alignment direction of the nozzles, and the printhead is scanned a plurality of number of times while ink droplets are discharged from different nozzles, thereby printing a predetermined printing region, comprising: convey means for conveying the printing medium in a convey direction by a predetermined convey amount every scanning; and control means for controlling the convey amount of the every scanning to a convey amount corresponding to either one of even and odd multiples of the nozzle pitch, and setting a convey amount corresponding to each of the even and odd multiples of the nozzle pitch at least once in the plurality of scanning operations. To achieve the above object, a control method for an image printing apparatus according to another aspect of the present invention has the following steps. That is, a control method for an image printing apparatus which prints an image by multipass printing in which a printhead having a plurality of nozzles that are aligned at a predetermined nozzle pitch and discharge ink droplets is scanned on a printing medium in a direction cross to an alignment direction of the nozzles, and the printhead is scanned a plurality of number of times while ink droplets are discharged from different nozzles, thereby printing a predetermined printing region, comprising: the convey step of conveying the printing medium in a convey direction by a predetermined convey amount every scanning; and the control step of controlling the convey amount of the every scanning to a convey amount corresponding to either one of even and odd multiples of the nozzle pitch, and setting a convey amount corresponding to each of the even and odd multiples of the nozzle pitch at least once in the plurality of scanning operations.
To achieve the above object, a computer-readable storage medium according to another aspect of the present invention has the following codes. That is, a computer-readable storage medium which stores a control program for an image printing apparatus which prints an image by multipass printing in which a printhead having a plurality of nozzles that are aligned at a predetermined nozzle pitch and discharge ink droplets is scanned on a printing medium in a direction cross to an alignment direction of the nozzles, and the printhead is scanned a plurality of number of times while ink droplets are discharged from different nozzles, thereby printing a predetermined printing region, the control program comprising: a program code of the convey step of conveying the printing medium in a convey direction by a predetermined convey amount every scanning; and a program code of the control step of controlling the convey amount of the every scanning to a convey amount corresponding to either one of even and odd multiples of the nozzle pitch, and setting a convey amount corresponding to each of the even and odd multiples of the nozzle pitch at least once in the plurality of scanning operations.
To achieve the above object, a control program according to still another aspect of the present invention has the following codes. That is, a control program for an image printing apparatus which prints an image by multipass printing in which a printhead having a plurality of nozzles that are aligned at a predetermined nozzle pitch and discharge ink droplets is scanned on a printing medium in a direction cross to an alignment direction of the nozzles, and the printhead is scanned a plurality of number of times while ink droplets are discharged from different nozzles, thereby printing a predetermined printing region, comprising: a program code of the convey step of conveying the printing medium in a convey direction by a predetermined convey amount every scanning; and a program code of the control step of controlling the convey amount of the every scanning to a convey amount corresponding to either one of even and odd multiples of the nozzle pitch, and setting a convey amount corresponding to each of the even and odd multiples of the nozzle pitch at least once in the plurality of scanning operations.